The calibration of an X-ray machine is important in diagnostic radiology. The measurement of the potential applied to an X-ray machine has been recognized as an important variable in the production of high quality diagnostic X-ray films. In the United States, the Radiation Control for Healthy and Safety Act of 1968 became law in 1973. The main intent of the law was to protect the population from unnecessary radiation exposure. One way to accomplish this is to reduce the number of retakes of X-rays. The law requires that X-ray machines meet certain requirements. One of these requirements is that the maximum applied input voltage, sometimes referred to as the peak kilovoltage (kVp), applied to the X-ray machine fall within certain limits specified by the manufacturer. If an X-ray machine is inaccurately calibrated, this may result in shortened component life and poor quality X-rays, which may result in retakes. Consequently, there is a need to periodically check the accuracy of the kVp setting on X-ray machines and recalibrate when required.
Diagnostic X-ray machines operate at relatively high voltages, such as on the order of 50 kV to 150 kV. Direct measurement of such a high voltage may be dangerous and has in the past been accomplished by disconnecting the high voltage circuits and reconnecting a high voltage divider having two large value resistance sections connected between the anode of the X-ray generator and ground and between the cathode of the generator and ground. The high voltage divider circuit is typically large in volume and size and the operation for measuring the high voltage in such apparatus is time-consuming and only qualified service personnel could accomplish this task. Hospital staff people have not normally been employed for conducting this test because of the size and weight of the divider circuit and the inherent danger involved in making such a measurement.
Alternatives to the direct measurement, utilizing a high voltage divider as discussed above, are various noninvasive measurement techniques presently being employed. This includes the use of a noninvasive film cassette, as well as a noninvasive electronic device employing filters and sensors. These noninvasive techniques measure the input voltage to an X-ray machine from measurements of the radiation the machine emits.
The film test cassettes (sometimes known as the Adrian Crooks or Wisconsin test cassette) have been used to determine the input kilovoltage to a radiation source from the measurements of the radiation it emits. A test cassette is placed in the field of an X-ray beam and operates on the principle that the extent of attenuation of an X-ray in a material, such as copper or aluminum, is related to the kilovoltage applied to the X-ray tube. X-ray film is exposed to X-rays that have been attenuated while passing through multiple layers of material including a copper sheet and a sheet that includes copper disks and holes. The measurement requires the assistance of skilled technicians, development of the film and reading of the film with a densitometer. The accuracy of this method is on the order of .+-.5 kV. Moreover, since such a test cassette can measure only the effective or average kV and not the true peak of the waveform, results will not reveal significant ripple or spiking on the waveform.
Another noninvasive device for measuring input voltage supplied to an X-ray machine takes the form of an instrument known in the art as a kVp meter. Examples of such meters are disclosed in various U.S. patents, including the patents to Zarnstorff et al., U.S. Pat. No. 4,697,280, Siedband, U.S. Pat. No. 4,361,900, as well as products manufactured by Keithley Instruments, Inc. as model Nos. 35070 and 35080. In general, these kVp meters operate on the principle of passing an X-ray beam through a pair of copper filters positioned side-by-side so that the X-ray beam is attenuated as it passes through each filter. The two filters are of different thicknesses and, hence, as the radiation passes through each filter, it is attenuated differently. The attenuated radiation from each filter is then detected by a pair of X-ray detectors, such as solid state photodiodes, which provide output electrical signals having magnitudes which depend upon the attenuated radiation levels from the two filters. A ratio of these two signals is then made. This ratio will vary with the input kilovoltage applied to the X-ray tube. The X-rays passing through the thicker material increase faster with increasing input kilovoltage than the X-rays passing through the thinner material. Consequently, the ratio of the signals representative of radiation passed through the thick material to that of the thin material starts at zero and increases as the kilovoltage increases. For very large kilovolts, the ratio approaches unity. These kVp meters typically operate over a range from 50 to 150 kV.
Recently, there has been significant interest dealing with mammography. This is the X-raying of the female breast to locate cancer at an early stage. Unlike a typical diagnostic X-ray machine, which operates in the range of 50 kV to 150 kV, the mammographic X-ray machines operate at a somewhat lower voltage level on the order of 25 kV to 40 kV. Another significant distinction is that the mammographic X-ray machines usually employ molybdenum anodes as opposed to the tungsten anodes which are use in diagnostic X-ray machines operating in the range of 50 kV to 150 kV. The use of molybdenum anodes for these lower voltage mammographic X-ray machines presents problems in attempting to measure the operating voltage with the typical kVp meters discussed hereinabove.
It has been determined that the photon spectrum for molybdenum in the mammographic region differs substantially from that of tungsten. Thus, in this region the photon spectrum for tungsten is a somewhat smooth inverted U-shaped curve, whereas that for molybdenum has a substantial discontinuity near the K edge of the anode material (approximately 20 kilovolts for molybdenum). Moreover, such a molybdenum anode will fluoresce at discrete energies on the order of 17.5 kV and 19.5 kV. Also, it is customary to employ additional filters made of molybdenum in a molybdenum X-ray machine which causes further suppression in the higher energy spectrum. As a consequence the ratio technique employed by the kVp meters, as discussed above, does not provide an adequately accurate measurement of the operating voltage of such mammographic X-ray machines.
The present invention is directed toward determining the operating voltage of an X-ray machine with an accuracy that is independent of the anode material. Thus, in the example given, the measurement is independent of whether the anode material is molybdenum or tungsten.
The present invention is based on the recognition that a chemical element, such as molybdenum or tungsten, exhibits an absorption phenomenon. Such elements when irradiated by an X-ray beam will absorb radiation at a predictable rate until the voltage applied to the X-ray machine attains a particular level and then a sudden transition takes place in the absorption rate. This transition is a sharp increase in the absorption rate and it corresponds with what is known as the K absorption edge of that particular chemical element. The K absorption edge refers to the K quantum shell. An electron can be removed from the K shell by photoelectric absorption. This takes place when photons of a sufficiently high energy level are incident upon an atom causing an electron to be ejected from the K shell. The threshold photon energy to achieve this is known as the K-absorption edge. Similar discontinuities are present in the L quantum shell as well as in the M quantum shell. However, elements have only a single sharp transition absorption edge in the K quantum shell. On the other hand, elements exhibit multiple absorption edges in the L quantum shell and in the M quantum shell. It would be difficult to determine from such multiple transitions the correct level of photon energy required to achieve the transitions. For this reason, it is believed that a more accurate determination of the photon energy level required can be made from sensing only the K absorption edge.
The patent to G. R. Harris et al., U.S. Pat. No. 3,766,383 discloses an apparatus for calibrating the kilovoltage of a diagnostic X-ray generator by placing a chemical element or test sample, having a known K-absorption edge, within an X-ray beam. The test sample is disposed at an angle of approximately 45 degrees to the generated radiation path so that some energy is reflected as scattered energy, and some energy is transmitted through the sample as transmitted energy. The scattered energy and the transmitted energy are detected and a ratio is calculated as to the transmitted and scattered detected radiation values. When this ratio changes significantly, it is indicative that the K-edge has been reached. Since the sample has a known K-absorption edge, this information is then used to determine the kilovoltage level.
The system proposed by Harris is awkward in its implementation. Because both the scattered as well as transmitted X-rays are detected, the detectors themselves must be positioned in different planes, one located in a plane above the test sample, and one located in a plane below the test sample. The structure to accomplish this would be relatively expensive and cumbersome in its implementation. In addition, the Harris system proposes the monitoring of the detector ratio as a function of the kilovolts applied, and this takes the form of an inverted V-shaped curve with an upsloping ramp which reaches a peak at the K-absorption edge of the test sample, and then a downward slope after the K-absorption edge has been exceeded. Consequently, the kilovoltage is a double valued function of the detector ratio. That is, there are two kilovolt levels for each detector ratio level, and, hence, for a single exposure or single reading, the operator would not know if the kilovoltage level at that ratio level is above or below the K-absorption edge.